A knowledge of the molecular events involved in the formation and repair of chromosome aberrations is fundamental to the understanding of several biological phenomena, including mutation, malignant transformation, loss of cell viability, and birth defects. A better knowledge of these events is also important in the rational design of combination chemo- and radiotherapy programs for the treatment of malignancy. Until recently, such understanding has been hampered by the absence of sensitive techniques to examine how specific DNA lesions are transformed into chromosome aberrations. The long range purpose is to elucidate the molecular mechanisms of chromosome aberration formation in human cells by the simultaneous monitoring of induction and repair of specific DNA lesions with that of chromosome lesions. This project combines the use of premature chromosome condensation (to directly measure chromosome damage and repair in interphase cells), alkaline and neutral DNA elution (to study DNA damage), CsCl density gradients (to isolate parental and newly replicated DNA), DNA repair inhibitors and inhibiting conditions, DNA repair deficient human and rodent cell lines, and drugs and radiation which induce specific types of DNA lesions. Four general interrelated experimental approaches are proposed and include the following: (1) A comparison of DNA and chromosome break repair kinetics after treatment of repair proficient and deficient quiescent cells with direct- and indirect-acting clastogens (in the presence or absence of repair inhibitors); (2) Determination of the role of the repair of chromosome breaks in the formation of chromosome exchanges; (3) An investigation of the fate of DNA-DNA crosslinks during DNA synthesis and the role of recombinational mechanisms in the formation of chromatid breaks and exchanges; and (4) Determination of the role of inhibiting the function of Topoisomerase II (e.g. by m-AMSA treatment) on the formation of chromosome aberrations and sticky chromosomes. The information generated by these studies will have significant implications in several health-related areas, including the molecular action of chemotherapeutic agents, the development of new rationales for combination chemotherapy in the treatment of malignancy, and the long term effects of clastogenic agents on cellular genomic integrity.